The present invention relates to improvements in the preparation of thermoplastic or elastomeric material onto which silane or a silane compound is grafted to obtain subsequently crosslinking in the presence of moisture.
It is known to graft silane or a silane compound onto the macromolecules of a thermoplastic or an elastomeric material being the base material. The base material, the silane additives, such as peroxide activators etc., are mixed and mechanically worked (kneded, extruded) before and, possibly, during grafting (see e.g. U.S. patent application Ser. No. 557,108; filed Mar. 10, 1975, now U.S. Pat. No. 4,289,860. Subsequently, the material is degassed (see e.g. U.S. patent application Ser. No. 642,764; filed Dec. 22, 1975, now U.S. Pat. No. 4,117,063), and e.g. extruded onto and around a cable, conductors, tubes or the like, which operation is followed by crosslinking.
Condensation crosslinking is, of course, well known. It is also known to graft organic functional alcoxy silane onto polyethylene macromolecules whereby so-called radical initiators such as peroxides are used to obtain the grafting proper. In cases mechanical working can be used to produce intermediary free radicals (see e.g. German printed patent application No. 1,794,028). It is also known to use as the base material a synthetic rubber such as a copolymer of ethylene vinylacetate (EVA) for condensation crosslinking. Unlike the case of thermoplastics, crosslinking of such an elastomeric material is produced primarily for producing a heat resisting material. However, such a material has inferior electrically insulative properties than polyethylene or its copolymers so that its use is linked to low voltage insulation.